The present invention is directed to a method of keeping inductor spouts, downgates or ingates, and outlet channels or pouring spouts and the like free of deposits while a cast iron melt is treated with pure magnesium in a casting process. The method is used for the production of cast iron with spheroidal or nodular graphite or vermicular graphite. The invention also includes the vessels for carrying out the method.
The conversion of an iron melt into cast iron with nodular or spheroidal graphite or into cast iron with vermicular graphite is achieved by treating the melt with magnesium or rare earth metals such as Ce, Ba, Ca or the like. It is known that magnesium has a high vapour pressure, low melting and boiling temperatures and a low specific gravity. Such characteristics lead to the use of magnesium, as a rule, as a preliminary or master alloy, such as FeSiMg with low Mg content. The magnesium content can vary between 5-30 percent by weight. The use of pure magnesium is possible only in special devices such as the pure magnesium converter.
It is also known that magnesium has a high affinity for oxygen and sulfur. Because of these characteristics and the low solubility of magnesium in the melt, the modifying action of magnesium on the graphite structure is effective only for a limited time period. Accordingly, magnesium is consumed by the reaction with the sulfur present in the melt, by oxidation due to oxygen in the atmosphere, as well as by reduction of the oxides present in the iron, in the slag and in the refractory materials contacting the melt. Therefore, a significant portion of the magnesium introduced into the melt is ineffective for the modification of the graphite. To slow down these reactions (so-called "fading") and to reduce the temperature loss of the melt, a channel type pressure furnace with an inert gas atmosphere was developed. Such a furnace is generally used as a temperature holding casting furnace.
In such a furnace, the fading effect caused by atmospheric oxygen and by evaporation of the magnesium is substantially reduced by the action of inert gas on the melt surface.
The use of master alloys contributes to a reduction of the magnesium activity. Other elements such as Fe, Si, Ni and the like are mixed with the melt. Accordingly, the reaction speed is reduced and the reaction between magnesium and sulfur is also slowed down with the result that the sulfur content cannot be substantially reduced. Thus, the degree of desulfurization is low and the reaction between free sulfur and magnesium is continued after the treatment whereby there is a quick reduction in the active magnesium content in the melt (fading). This process is not influenced by the presence of an inert gas atmosphere.
Treatments with a master alloy based on FeSi develop acid reaction slags containing more than 60% of oxides which are easily reducible by means of magnesium, such as FeO, MnO, and SiO.sub.2. Even after removal of the reaction slag from the surface of the melt, a certain portion of the easily reducible oxides remains suspended in the melt. Accordingly, the reaction, that is, oxidation, Mg+S and the like is continued, and additional reaction products are formed.
In addition to acceleration of magnesium fading, the slag also deposits or settles out at certain places in the furnace and causes operational problems, such as blockages in the inlet and outlet casting channels and inductors spouts. Such deposits lead to considerable furnace maintenance costs, a rapid decay of the magnesium and a decrease in the lifetime of the furnace lining.